Method of grain refining cast magnesium alloy

ABSTRACT

A method of grain refining cast magnesium alloy includes adding to a magnesium alloy melt containing aluminum and manganese, pure carbon powder, or a carbon source in combination with niobium pentoxide or vanadium pentoxide.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of refining the grains of thecast magnesium alloy without generating dioxin to improve the mechanicalproperties of the magnesium alloy.

2. Description of the Prior Art

Methods of grain refining magnesium alloy containing aluminum, such asAZ magnesium alloy, include methods that do not require a grain refinerand methods that require a grain refiner.

The former is a superheating method in which the casting melt isprepared by heating the alloy to around 150 to 250° C. (1123 to 1173 K)above the melting point, maintaining it at that temperature for 5 to 15minutes (300 to 900 seconds), and then rapidly cooling it to the castingtemperature. The grain refining mechanism is said to be heterogeneousnucleation by an Al—Mn—Fe compound. Because of the high processtemperature, the energy costs of the method are high, and there is alsothe expense involved in preventing oxidation of the melt and in castingladle checking and maintenance procedures. Thus, the method is beset byproblems of economic feasibility and safety.

The latter includes a carbon addition method in which acarbon-containing compound is added to the melt at around 750° C. (1023K). The grain refining mechanism is said to be heterogeneous nucleationby aluminum carbide (Al₄C₃) produced by carbon in the compound reactingwith aluminum in the melt. In commercial processes C₇Cl₆ is used to beadded as a grain refiner, but this is no longer allowed because itproduces dioxins (2,3,7,8-tetrachlorodibenzo p-dioxinCl₂(C₆H₂)O₂(C₆H₂)Cl₂).

There is also the ferric chloride method (Elfinal method) in whichferric chloride (FcCl₃) is added to a melt at around 760° C. (1053 K)and the melt is maintained for 30 to 60 minutes (1800 to 3600 seconds),giving rise to Al—Mn—Fe compound heterogeneous nuclei that are said toproduce the grain refinement. It has been reported that in order toobtain a pronounced refinement effect, the Mn content has to be above acertain value. The problem with this method is corrosion produced by alocalized battery effect of the Fe and My.

Compared to the superheating process, grain refinement by adding a grainrefiner has the merits of a lower process temperature and suitabilityfor large-volume melts. But, it also has the problem that it producesdioxin, generating a need for a refining agent that can be used insteadof C₂Cl₆.

Also, in the case of the ferric chloride method, corrosion resistance isdegraded by the battery effect, so there is a need to replace the FeCl₃with a substance that alters the structure of the Al—Mn compound toeffect grain refinement of castings without loss of corrosionresistance.

An object of the present invention is to provide a method of grainrefining cast magnesium alloy to improve the mechanical properties ofthe alloy without producing dioxin or degrading the corrosionresistance.

SUMMARY OF THE INVENTION

To attain the above object, the present invention provides a method ofgrain refining cast magnesium alloy comprising adding, as a grainrefiner, (i) pure carbon powder or (ii) a carbon source in combinationwith niobium pentoxide (Nb₂O₅) or vanadium pentoxide (V₂O₅) to amagnesium alloy melt containing aluminum and manganese.

By using a grain refiner thus comprised of carbon powder alone or acarbon source in combination with niobium pentoxide or vanadiumpentoxide, it is possible to refine, without producing dioxin, the graindiameter of cast materials to 100 μm or smaller compared to graindiameters in the order of 140 to 200 μm when a refiner is not used, alsoimproving the mechanical properties of the cast materials.

Particularly, in the case of addition of a carbon source in combinationwith niobium pentoxide or vanadium pentoxide, there can be obtained aneffect of magnesium alloy refinement and an effect of shaping the Al—Mncompound into spheres.

Further features of the invention, its nature and various advantageswill be more apparent from the accompanying drawings and followingdetailed description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an apparatus used in Example 1 of the presentinvention.

FIG. 2 is a graph showing the relationship in Example 1 between thetemperature, at which 5-μm carbon powder is added, and the average grainsize of the cast alloy structure.

FIG. 3 is an optical micrograph of the cast alloy structures obtained inExample 1 by adding 5-μm carbon powder at different temperatures.

FIG. 4 is an optical micrograph of the cast alloy structures obtained inExample 1 by adding 5-μm carbon powder for different times at atemperature of 1023 K.

FIG. 5 is an optical micrograph of the cast alloy structures obtained inExample 1 by adding 5-μm carbon powder for different times attemperatures of 1053 K and 1073 K.

FIG. 6 shows an apparatus used in Example 2 of the present invention.

FIG. 7 is a flow chart of the process of Example 2.

FIG. 8 is a graph showing the relationship in Example 2 between thetemperature, at which Nb₂O₅ is added, and the effect on the averagegrain size.

FIG. 9 is an optical micrograph of the cast alloy structures obtained inExample 2 by adding Nb₂O₅ at different temperatures.

FIG. 10 is a graph showing the relationship in Example 2 between theamount of Nb₂O₅ added at 1033 K, 1053 K and 1073 K and the effect on theaverage grain size.

FIG. 11 is an optical micrograph of the cast alloy structures obtainedin Example 2 by adding different amounts of Nb₂O₅ at temperatures of1033 K and 1053 K.

FIG. 12 is an optical micrograph of the cast alloy structures obtainedin Example 2 by adding different amounts of Nb₂O₅ at a temperature of1073 K.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention relates to a method of grain refining castmagnesium alloy comprising adding (i) pure carbon powder or (ii) acarbon source in combination with niobium pentoxide (Nb₂O₅) or vanadiumpentoxide (V₂O₅) to a magnesium alloy melt containing aluminum andmanganese. This addition can shape an Al—Mn compound into spheres andimprove the mechanical strength of the cast magnesium alloy.

There is no particular limitation on the magnesium alloy containingaluminum and manganese, so long as it contains aluminum as a componentand manganese as an impurity. For example, it is possible to use AZ91,which is also used for sand mold casting. Concerning the pure carbonpowder used as a refining agent, in Example 1 described below, graphitewith a particle size of up to 5 μm is used with a ultrahigh-purity argon(Ar) gas carrier, but this is not limitative. For example, helium (He)can be used as the carrier gas, and the finer the graphite is, thebetter the result. Also, activated carbon can be used on its own.

The amount of the pure carbon powder added as a refining agent that canexert the refinement effect is small as much as around 0.005 to 0.5% byweight based on the amount of the magnesium alloy melt.

The temperature of 993 K or higher at which the pure carbon powder isadded to the molten Mg alloy will suffice. The more elevated above 1053K the temperature is, the shorter the time it takes to achieve therefinement. However, since too high a temperature can result in ignitionof the molten material, around 1023 K is preferable.

When a carbon source and Nb₂O₅ or V₂O₅ are used as the refiner, since itis assumed that the carbon source will be used in conjunction with theNb₂O₅ or V₂O₅, it is not necessary to use the aforementioned pure carbonpowder. Carbon dioxide (CO₂) gas or the like, or solid activated carboncan be used. When carbon dioxide (CO₂) gas is used an a carbon source,addition of sulfur hexafluoride (SF₆) gas or fleon 134a (HFC-134a)enhances the grain refinement effect. The Nb₂O₅ or V₂O₅ added with thecarbon source can be added in powder form, or as tablets, pellets orother such aggregated forms. When activated carbon or the like is usedas the carbon source, it too can be combined for addition in anaggregated form.

It is preferable to add the Nb₂O₅ or V₂O₅ in an amount that is 0.1 to 3%by weight based on the melt amount. If the added amount is less than0.1% by weight, the grain refinement effect attained will not besufficient. Thus, using a mixture of carbon dioxide (CO₂) gas withsulfur hexafluoride (SF₆) gas or fleon 134a (HFC-134a) or usingactivated carbon on its own will provide a sufficient grain refinementeffect. Conversely if the added amount exceeds 3% by weight, the resultis a higher impurity content without any additional refinement effect,degrading the mechanical properties of the cast product thus obtained.

It is preferable for the Mg alloy melt to be at a temperature of 993 to1073 K, and more preferably 1033 to 1073 K, when the Nb₂O₅ or V₂O₅ isadded. A relatively higher temperature increases the grain refinementeffect. Grain refinement is not sufficient when the temperature is lowerthan 933 K. Also, when the temperature is higher than 1073 K, energycosts become high without any additional grain refinement. Inparticular, it was found that a high grain refinement effect wasobtained at an adding temperature of 1073 K, regardless of the amount ofNb₂O₅ or V₂O₅ added; that is, high grain refinement was obtained evenwith the minimum 0.1 wt % addition.

As described above, in the case of addition of a carbon source incombination with Nb₂O₅ or V₂O₅, there can he obtained an effect ofmagnesium alloy refinement and an effect of shaping the Al—Mg compoundinto spheres.

When no grain-refining agent is used, the size of cast grains is around140 to 200 μm. In accordance with the present invention, pronouncedgrain refinement was obtained. In the case of this invention, a castgrain size of 100 μm or smaller was set as a target signifying theattainment of a sufficient grain refinement effect. Moreover, castproducts in which the grains were refined to 100 μm or smaller were alsoobserved to contain spheroidized Al—Mn compounds diffused within thegrains, which can be expected to improve the mechanical properties.

Thus, in accordance with the present invention, by using a grain refinerthus comprised of carbon powder alone or a carbon source in combinationwith niobium pentoxide or vanadium pentoxide, it is possible to refinethe cast alloy grains to 100 μm or smaller, and improve the mechanicalproperties.

Examples of the present invention will now be described. However, it isto be understood that the invention is not limited to the examplesdescribed below.

EXAMPLE 1

A cylindrical melting pot was fabricated by bending and gas-weldingFe—Cr system SUS 430 stainless steel (Fe-18%Cr) plate not containing Ni.To increase the resistance to high-temperature oxidation, the meltingpot was plated by immersion in a melt of pure aluminum, andsuperheat-diffused to form a surface layer of Mg and low-wettabilityFeAl₃. The melting pot and all casting utensils were coated with specialreagent-grade magnesium oxide to prevent the admixture of impuritieswhen the alloy is melted.

In this example, commercial AZ91E magnesium alloy was used, having thecomposition shown in Table 1 below, in which the unit is “mass %.”

TABLE 1 Al Zn Mn Si Cu Ni Fe Mg 9.01 0.82 0.22 0.01 0.001 0.0002 0.0017Bal.

After pickling in nitric acid to remove impurities, the magnesium alloyingots were placed in the melting pot and melted in an electric furnace.FIG. 1 shows the apparatus used, which comprises a cylinder 1 ofultrahigh-purity argon gas, a unit 2 for spraying carbon powder, a tank3 of carbon powder having a particle size of 5 μm, a 200-mesh wirescreen 4, the electric furnace 6, and the melting pot 5. Via the unit 2,gas from the cylinder 1 is supplied in pulses to the tank 3, blowing thecarbon powder through the screen 4 and into the Mg alloy being melted inthe melting pot 5 in the furnace 6.

700 grams of pickled alloy ingot was placed in the melting pot formelting. After melting, the alloy was maintained at 973 K while theargon gas intermittently blew the 5-μm carbon powder into the melt for600 seconds. The alloy was then cast and an optical microscope was usedto measure the average size of the cast grains. The same procedure wasused to add 5-μm carbon powder to melts maintained at 993 K, 1013 K,1053 K and 1073 K, respectively, and the average cast grain size in eachcase was measured.

For comparison, average grain sizes were measured in respect of alloycastings made after melting the alloy ingots, and alloy castings madeafter being subjected to a flow of just argon gas for 600 seconds.

FIG. 2 shows the relationship between the temperature at which 5-μmcarbon powder was added and the average grain size of the cast alloystructure, when the carbon powder was added for 600 seconds, and FIG. 3is an optical micrograph of the cast alloy structure. FIG. 2 shows thatthe average grain size was around 138 μm in the case of untreated alloyand that a refinement effect was observed when the temperature ataddition was at least 1000 K. Grain refinement was particularlypronounced when the temperature at addition was 1023 K or above, andmore so at 1053 K or above, with grains being refined to 70 μm or below.This marked effect can also be seen in the cast structure micrograph ofFIG. 3.

Next, castings were produced by maintaining the Mg alloy melt in themelting pot at 1023 K while adding 5-μm carbon powder for 300, 600, 900,1200, 1500 and 1800 seconds, respectively, and the average grain size ineach case was measured. FIG. 4 shows the results. From FIG. 4, it can beseen that a refinement effect was observed when the adding time was atleast 600 seconds, and a pronounced refinement effect was observed whenthe adding time was 900 seconds or more.

Finally, 5-μm carbon powder was added for 300, 600 and 900 seconds inrespect of Mg alloy melts in the melting pot being maintained at 1053 Kand 1073 K, and the average grain size in each case was measured. Theresults are shown in FIG. 5, from which it can be seen that a refinementeffect was observed even at an adding time of 300 seconds, and aparticularly pronounced refinement effect was observed when s an addingtime of 600 seconds or more was used.

EXAMPLE 2

A stainless-steel melting pot was fabricated, as in Example 1. The samematerial was also used to fabricate a chamber to prevent combustion, anda phosphorizer for adding Nb₂O₅, As in Example 1, AZ91E magnesium alloywas used, and the apparatus of FIG. 6 was used to perform casting by theprocedure listed in FIG. 7. The cast grains thus obtained were measuredand analyzed using a scanning electron microscope (SEM). In FIG. 6,reference numeral 5 denotes the melting pot, numeral 6 an electricfurnace, numeral 7 a temperature controller, numeral 8 a pen recorder,numeral 9 a thermocouple used to measure the temperature inside thefurnace, and numeral 10 a thermocouple used to measure the temperatureof the melt in the melting pot.

First, in order to investigate the effect that the temperature has onthe Al—Mn compound-spheroidizing effect of Nb₂O₅, the amount of Nb₂O₅added (0.5% by weight) was kept the same while just the temperature ofthe melt was varied. Further, the alloy melt was sprayed with a mixedgas of CO₂ gas and SF₆ gas for about 900 seconds, with the CO₂ gas usedas a carbon source Specifically, using the phosphorizer, tabletizedNb₂O₅ was added in an amount of 0.5% by weight of the melt amount, inrespect of melts maintained at each of the temperatures 993, 1013, 1055and 1073 K. After the completion of the reaction in each case, the alloywas removed from the furnace and allowed to cool in air. At 973 K, thealloy was poured into molds (at room temperature) to form round bars 20mm in diameter and 100 mm in height. For comparison, an alloy ingot wasalso melted and cast. An optical microscope was used to measure theaverage size of the cast alloy grains.

FIG. 8 is a graph showing the relationship between the temperature atwhich Nb₂O₅ is added and the effect on the average grain size. The castalloy structures obtained by an optical microscope are shown in FIG. 9.The average grain size in the case of untreated material was 192 μm. Ascan be seen from FIG. 8, grains were finer than that in each case, andin the case of the temperatures 1033, 1055 and 1073 K, the grainrefinement effect was particularly pronounced, with grains measuring 100μm or smaller. That is, grain refinement shows a tendency to increasewhen the addition temperature is higher. A high refinement effect wasobserved when the mixed gas (CO₂+SF₆) used as the carbon source athigher temperatures that promoted the reduction reaction. Also, a highrefinement effect was observed when the Nb₂O₅ was added at highertemperatures that promoted the reduction reaction, and spheroidizationof the Al—Mn compound was markedly observed. As a result, 1033 K or morewas found to be the advantageous temperature that could bring about aspheroidizing effect.

Next, alloys were cast in the same manner as mentioned above, with 0.1,0.2, 0.5 and 1.0 wt % Nb₂O₅ added to melts maintained at each of thetemperatures (1033, 1053 and 1073 K) at which the grain refinementeffect and Al—Mn compound-spheroidizing effect were observed. Thematerial was non-balancedly solidified to carry out the observation.Therefore, solution heat treatment at 673 K for 14400 seconds wascarried out to dissolve eutectic crystals produced by the non-balancedsolidification. The section method was used to measure average grainsize by an optical microscope.

FIG. 10 is a graph showing the relationship between the addingtemperature and the amount of Nb₂O₅ that is added and has the effect onthe average grain size when the temperature at the time of the additionis 1033 K, 1053 K and 1073 K, respectively FIGS. 11 and 12 are opticalmicrographs of the cast alloy structures thus obtained.

From FIG. 10, it can be seen that in the case of 1033 and 1073 K, theaverage grain size was refined, and s pronounced spheroidization ofAl—Mn compounds was observed regardless of the amount of Nb₂O₅ added. Ataround 60 μm, grain refinement was particularly pronounced in the caseof 1073 K.

The present invention has been described in the foregoing with referenceto examples. However, it is to be understood that the invention is notlimited to the above examples, and can be practiced using configurationsmodified to the extent that such changes do not depart from the scope ofthe appended claims.

The method of grain refining cast magnesium alloy in accordance with thepresent invention makes it possible to refine cast grains and improvemechanical properties, without producing dioxin or degrading corrosionresistance.

What is claimed is:
 1. A method of grain refining cast magnesium alloycomprising: adding a carbon source in combination with niobiumpentoxide, Nb₂O₅, or vanadium pentoxide, V₂O₅, to a magnesium alloy meltcontaining aluminum and manganese, wherein the niobium pentoxide orvanadium pentoxide is added to the melt in an amount that is from 0.1 to3% by weight of an amount of the melt at a temperature of from 993 to1073 K.
 2. The method according to claim 1, wherein the pure carbonpowder is added to the melt by a carrier gas.
 3. The method according toclaim 2, wherein the carrier gas comprises at least one member selectedfrom the group consisting of argon and helium.
 4. The method accordingto claim 2, wherein the carrier gas comprises argon.
 5. The methodaccording to claim 2, wherein the carrier gas comprises helium.
 6. Themethod according to claim 1, wherein the pure carbon powder is added tothe melt in an amount is from 0.005 to 0.05% by weight of an amount ofthe melt at a temperature of from 993 to 1023 K.
 7. The method accordingto claim 1, comprising adding the carbon source to the magnesium alloymelt.
 8. The method according to claim 1, wherein the melt containsmanganese.
 9. The method according to claim 1, comprising adding thepure carbon powder to the magnesium alloy melt.
 10. The method accordingto claim 1, wherein the melt contains aluminum.
 11. The method accordingto claim 1, wherein the pure carbon powder has a particle size less thanor equal to 5 μm.
 12. The method according to claim 1, wherein theniobium pentoxide, Nb₂O₅, or vanadium pentoxide, V₂O₅, is added to themelt in at least one form selected from the group consisting of apowder, tablet and pellet.
 13. The method according to claim 1, whereinthe niobium pentoxide, Nb₂O₅, is added to the melt in at least one formselected from the group consisting of a powder, tablet and pellet. 14.The method according to claim 1, wherein the vanadium pentoxide, V₂O₅,is added to the melt in at least one form selected from the groupconsisting of a powder, tablet and pellet.